Nickel has a marked effect on the colour of Cu-Ni alloys. The copper colour becomes lighter as nickel is added. Alloys are almost silvery white from about 15% nickel. The lustre and purity of the colour increases with nickel content; from about 40% nickel, a polished surface can hardly be distinguished from that of silver.
The density of copper (8.93 kg/dm3at 20 °C) varies only slightly with increasing nickel content (density of nickel at 20 °C = 8.9 kg/dm3) and is 8.9 kg/dm3 for all Cu-Ni alloys specified in DIN 17 664.
The high thermal conductivity of pure copper of 394 W/(m.K) is severely reduced by nickel; it reaches a minimum of c. 21 W/(m.K) at about 45% Ni. The coefficient of linear expansion initially decreases sharply with addition of nickel, then more slowly. The specific heat (at 20 °C) of copper is 0.385 J/(g.K) and of nickel is 0.452 J/(g. K). As nickel content increases, it first diminishes slightly and a mean value of 0.377 J/(g.K) can be expected.
The electrical resistivity of Cu-Ni resistance alloys at different temperatures rises steeply with nickel content, so that Cu-Ni alloys are suitable as resistance materials. A maximum occurs at c. 45% Ni. The minimum of the temperature coefficient of electric resistance is in approximately the same concentration range.
The high thermoelectric power of Cu-Ni alloys in the range between 40 and 50% Ni is particularly noteworthy compared with other metals such as iron, copper, platinum etc. They are therefore especially suitable for use in thermocouples for temperature measurements in a moderate temperature range. The high thermoelectric force of CuNi44 excludes its use as resistance material in low-voltage appliances, because the copper connections to CuNi44 form a thermocouple.
Cu-Ni alloys do not exhibit any ferromagnetism. Copper is diamagnetic, nickel is ferromagnetic. Nickel-copper alloys change from diamagnetic via paramagnetic to ferromagnetic as nickel content increases. Depending on the alloy, iron has a small effect when it is present in solid solution. If iron is precipitated, these ferromagnetic microscopic particles lead to a macroscopic increase of ferromagnetism.
The precipitate-free matrix remains diamagnetic or paramagnetic. Cu-Ni alloys containing 20 to 25% Ni and 20% Fe or about 25% Co are pronounced magnetic materials. As a result of their high remanence and coercive force, they are also suitable for permanent magnets.
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TAG: Copper Nickel Alloy
